Myocardial infarctions, or heart attacks, are one of the commonest causes of serious illness and death in Western countries. The coronary arteries, which supply the heart with oxygen and nutrients, may become diseased with atherosclerosis that reduces blood flow. Atherosclerotic plaque within the artery may lead to the clotting of blood (thrombosis) in the artery, and this can cause sudden, complete obstruction and result in damage to a substantial area of heart tissue. This may end in sudden death, usually due to abnormal heart rhythm that prevents effective pumping.
An atherosclerotic plaque is the site of an inflammatory reaction within the wall of an artery. The plaque is made up of a core containing lipid and inflammatory cells surrounded by a connective tissue capsule. The portion of the capsule facing the arterial lumen is called the plaque cap. Atherosclerotic plaque rupture is the primary mechanism which leads to thrombus formation. When the plaque ruptures, the plaque cap tears to expose the lipid core to blood in the arterial lumen. The core area is highly thrombogenic, containing tissue factor, fragments of collagen, and crystalline surfaces to accelerate coagulation. The exposed collagen triggers platelets to adhere to the site. The aggregated platelets then initiate the blood clotting cascade and cause a thrombus to form. Thrombus forms initially in the plaque itself and is then expanded and distorted from within so that it extends into the arterial lumen. Certain atherosclerotic plaques, termed vulnerable plaques, are at greater risk of rupture than others. Such vulnerable plaques are characterized by, among other things, a large core, a thin cap, and a large number of inflammatory cells.
Plaque rupture is the result of enhanced inflammatory activity within the plaque which can be triggered by physical and chemical stresses. Portions of the coronary arteries which are surrounded by the ventricular myocardium are subjected to mechanical stresses when the ventricles contract during systole. Vulnerable plaques located in the intra-myocardial portions of coronary arteries may thus be at great risk for rupture.